We report in this Letter the outstanding frequency stability performance of an autonomous cryogenic sapphire oscillator (CSO) presenting a flicker frequency noise floor below 2 × 10 − 16 near 1000 s of integration time and a long-term Allan deviation limited by a random walk process of ∼ 1 × 10 − 18 τ. The frequency stability qualification at this level called for the implementation of sophisticated instrumentation associated with ultra-stable frequency references.
